The present invention relates to a semiconductor integrated circuit fabricated on a semiconductor substrate, and more particularly to a protective device for preventing an integrated circuit fabricated on the same substrate from destruction due to excessive electrostatic charges.
In order to prevent an active element such as a MOS field effect transistor and a bipolar transistor from destruction due to excessive electrostatic charge applied to an external terminal which is connected to the active element, a protective device is connected with the external terminal.
A typical example of a known protective device is composed of a resistor connected in series between the external terminal and an electrode of the active element such as a gate of a MOS transistor and a protecting MOS transistor connected between the electrode of the active element and a ground potential source with a gate connected to the ground potential source. The external terminal is formed by a bonding pad having a relatively large area and the resistor is formed by a diffusion region extending from the bonding pad to a drain of the protecting MOS transistor.
The diffusion region as the resistor is required to be formed with a relatively large length to obtain a predetermined resistance value. Accordingly, a relatively large area is required to fabricate the protecting device on a semiconductor substrate. Furthermore, if another diffusion region is formed adjacent to the diffusion region (resistor diffusion region) serving as the resistor, a parasitic bipolar transistor is produced therebetween, and when an excessive voltage is applied to the bonding pad and the resistor diffusion, an abnormal current flows between the resistor diffusion region and the above additional diffusion region. Therefore, a certain distance must be kept between the resistor diffusion region and the another diffusion region to avoid the above parasitic transistor. Thus, the further area on the semiconductor substrate have been required to arrange the protective device and a predetermined function circuit on the same substrate.